The present invention relates to loud speakers. More particularly, it relates to low frequency loud speakers for use adjacent to two mutually perpendicular, intersecting boundaries of a room, such as a floor and a wall, a ceiling and a wall or two walls.
For over sixty years loudspeakers have undergone an evolution in design beginning with a simple, electromechanically driven diaphragm invented by Chester W. Rice and Edward W. Kellogg reported in the Proceedings of the International Radio and Electronics Society in 1923. As the ability to record and preserve sounds electronically has improved, so has the need to transduce electrical impulses into sound waves with greater fidelity.
For accurate reproduction of musical sounds waves, a range of frequencies must be possible from a loud speaker without distortion or alternation of the original music by the resonant frequencies of the speaker itself.
Considering the full range of frequencies a human being can hear, from approximately 40 Hertz (Hz) to 15,000 Hz or nine octaves, the midrange frequencies between 1,000 Hz and 5,000 Hz are the most important. For example, humans hear best and speak at frequencies in the midrange; distortion of sounds in the mid range seems to irritate the listener more than distortion at the high or low end of audible frequencies.
The low frequencies, however, are the most difficult for a speaker to reproduce. It is known that the horn is an acoustic transformer whose transformer action creates a better impedence match between the driver and the air. This is extremely important due to the fact that the pitch of the tone decreases as the wavelength increases. For this reason, physical laws dictate a certain sound generating surface for adequate power output and efficient operation. When a diaphragm's physical dimensions are small in comparison to the wavelength being radiated, the acoustic power output of the source will be small.
The direct radiator loudspeaker used in a simple baffle must attempt to grab hold of all the air in direct contact with the diaphragm. The match between the diaphragm size and all the air is a poor one resulting in low efficiency. By using a horn the impedence match between the driver and the air at the mouth of the horn is improved because the mouth of the horn is in contact with a much larger surface of air.
An important means of compensating for limitations in the size of a diaphragm has been the use of a horn. A horn is simply a reasonably rigid barrier to a column of air through which sound waves move. However, to maintain a certain size sound generating surface, a horn's dimensions must approach a sizable fraction of the cutoff wavelength.
For a horn to successfully load a loudspeaker, certain conditions must be met. For a given low frequency response, the horn's throat size, length, flare rate and mouth size must be carefully chosen. The flare rate and the diameter of the horn mouth determine the lowest frequency at which the horn will operate. If the cross section of the horn perpendicular to its long dimension increases uniformly from throat to mouth so that the flare rate along the length of the horn is constant, the horn will transform the wave and radiate it more efficiently. If the cross sectional area of the horn increases exponentially with its length, the horn transforms sound waves very efficiently. Furthermore, the longer the exponential horn, the lower the frequency it can radiate; but the size of the mouth of an exponential horn increases rapidly, exponentially, with its length. Thus, the designer of an exponential horn for radiating low frequency sound waves must balance a speaker's lowest frequency response against the horn's ultimate length and the size of the horn mouth.
In the past large horns have been built for locations where space was not a limitation. Many of these systems occupy more than 100 cubic feet of space. In the home, however, space is usually unavailable for systems of this size. Usually, the volume limitation is further constrained by the size of a standard doorway through which the speaker must be moved.
However, it is possible to fold a horn so that the length is significantly reduced. Horns have been folded into complicated shapes to reduce their external dimensions or to increase length without creating an unmanagable horn. Bifurcating, or splitting the horn, often simplifies the internal shape of a folded horn.
Numerous loud speakers have been designed to incorporate a labyrinth or concentric cylinders or chambers of other shapes as a means for creating a folded path of great length and increasing cross section for radiating low frequencies. However, very long paths introduce a form of sound distortion called time-delay distortion, that is, distortion due to the separation of base and treble events because of the extra time it takes the low frequency sounds to travel through the horn and ultimately to the listener while the directly radiated mid and high frequencies produced at the same time have already arrived. A horn of about four feet in length is the maximum length the low frequencies should travel compared to the distance traveled by mid and high frequencies before time-delay distortion becomes noticeable.
Additionally, there have been several techniques employed to reduce the size of the horn needed to produce high fidelity sound. For example, placing the horn in the corner of a room takes advantage of the technique called imaging to enhance power output. By radiating sound waves from a corner, the diapragm radiates the wave into only one eight of the space of a speaker suspended in free space. The power of the sound wave is not dissipated over all space but only one eight because the sound is reflected off the three mutually perpendicular interfacing surfaces of a room. Furthermore, the three mutually perpendicular surfaces of a corner act as extensions of the corner-placed horn because they form diverging sound boundaries just as the horn is a diverging sound boundary. Corner placement is a very effective technique for reducing the size of the speaker or improving low frequency response by using the corner to confine the sound wave once outside the speaker and as an extension of the flare of the speaker horn.
However, although corner placement improves the low frequency response of a speaker, it affects the mid frequencies adversely. By moving a speaker away from a corner to the side of a room, mid-frequency response is improved. Furthermore, side placement improves the quality of the low frequency response because low frequency output is a more uniform function of frequency for a side-placed speaker.
Another technique is the use of an enclosure or back air chamber behind the sound-generating diaphragm to create a resistence of the air reacting against the backward movement of the diaphragm. This technique balances the air mass load focused on the front side of the diaphragm with an equivalent air mass provided by an appropriately sized back air chamber. This is very effective in reducing distortion associated with the non-linear motion of the diaphragm allowing the speaker to see a more resistive load, thereby increasing low frequency output.
If this enclosure is properly sized and vented, the sound waves from the back of the diaphragm can be directed out of the back air chamber to join in phase with the sound waves produced by the front of the diaphragm to further increase output.
There are numerous loud speaker designs. Typical of earlier designs was the labyrinth or maze such as disclosed in the patent of Mercurius (U.S. Pat. No. 2,277,525), Forrester (U.S. Pat. No. 2,646,852) and Pappanikolaou (U.S. Pat. No. 4,165,761).
Several designs have openable panels to allow the low frequencies to be directed into a room other than by direct radiation from the diaphragm front. Weil (U.S. Pat. No. 1,820,996) and Read (Pat. No. 2,805,729) have such panels, the former to be closed when the speaker is not in use and the latter to be closed to create an infinite backwave.
Corner placement has been advocated for year. See for example Stone's Sound Producing Device (Pat. No. 1,819,721). Two patents issued to Klipsch (U.S. Pat. Nos. 2,310,243 and 2,373,692) disclose speakers designed for corner placement. Pat. No. 2,310,243 shows an enclosure designed to back load the low frequency speaker by providing a horn receiving sounds from the back of the diaphragm and directing those sounds directly into the corner of a room. The sound continues to expand from the back of the speaker into the spaces between the sides of the horn and the side walls. Thus, the two intersecting walls and the floor, or ceiling, cooperate to extend the apparent size of the horn. The mid and high frequencies are radiated directly from the forward side of the diaphragm.
Pat. No. 2,373,692 discloses a loudspeaker also having a folded horn that directs sound into a room corner where it follows an expanding path between the speaker and the side walls of the room. With the exception of the horn outlet, the enclosure is sealed where the low frequency driver is mounted on the interior of the speaker enclosure with the front of the diaphragm connected to the horn throat projector via throat. The structure not only serves to form the horn boundary and a location for the driving unit within the speaker, but also provides an air chamber rearwardly of the diaphragm. This properly sized air chamber serves to offset the mass reactance of the throat impedence.
The Gillum and Klipsch patent (U.S. Pat. No. 4,210,723, discloses a large bifucated folded horn with a rear-facing diaphragm. An unvented back air chamber behind the sound generating diaphragm creates a resistence against the backward movement of the diaphragm. The back air chamber resistence balances the resistence of the air in the horn in front of the diaphragm. This loud speaker is designed to be used away from walls and in particular in auditoria on a stage with only the stage surface to help direct sound.
Many of the previous designs are for speakers for corner use where the mid range frequencies are adversely affected along with woofer output versus frequency. Many designs have very long sound paths but do not have the proper flare rate relative to the path length to transform the low frequency sound wave efficiently. Those that have horns are inefficiently designed and do not have the proper geometric relationship of throat to mouth. None takes advantage of the outside as well as the inside of the speaker to extend the apparent size of the horn. None uses a vented back air chamber to combine front and back waves for greater power output.
It is an object of the present invention to overcome these objections in the prior art. Specifically, it is an object of the present invention to project low frequency sound waves from a compact loud speaker positioned away from room corners without distorting the mid and high frequency sound waves. It is a further object of the invention to project undistorted low frequency sound waves with substantial and uniform power output through the operating range. It is a still further object of the invention to project sound waves from a rear facing diaphragm into a bifucated, folded exponential horn of sufficient length to produce efficiently sounds waves having a frequency as low as 40 Hz. It is still a further object of the invention to cover the frequency range from 40 Hz to 400 Hz with high fidelity. It is a still further object of the invention to use two mutually intersecting planes such a floor and a wall, a ceiling and a wall or two walls to cooperate with the speaker to extend the apparent length of the horn. It is a further object of the invention to use the inside and the outside of the speaker to extend the apparent length and mouth size of the horn. It is an object of the invention to use a vented back air chamber to radiate the diaphragm's back wave into the horn to further enhance power output. These and other object of the invention will be readily apparent from the description of the invention.